With the fans, it's like being in Death Valley in July.
Without fans, it's like having your head in a blast furnace.
I'd say that this is the one modification that everyone needs. It's hot in the armor anyway, so you'll do massive sweating. If you do not have some air circulation in there, it'll be intolerable for long periods of time and could be dangerous on long marches. Also, if you consider the sweating/breath factor, you're going to have even worse fogging issues than normal. Of course, this is speaking from experience living in Georgia (home of hot and muggy). I'd really suggest wiring in some computer fans. They don't take up a lot of room, don't need massive amounts of power to work, and they do wonders as little as they are.